In order to provide SMS functionality for User Equipment (UE) having a data connection to an Evolved Packet System (EPS) mobile telecommunication network, it has been suggested to use either so-called “native” solutions, i.e. the transport of short messages solely by using Packet Switched (PS) core network entities (or “SMS via EPS”), or a so-called Circuit Switched Fallback (CSFB) solution. The introduction of a native solution of a short message service in the Evolved Packet System (EPS) (SMS via EPS) has not been retained within the 3GPP standardization process. Alternative to a native Evolved Packet System (EPS) solution for the transport of short messages is to at least partly use Circuit Switched (CS) entities or components of the core network. Hereinafter, such use of Circuit Switched (CS) entities or components of the core network are also called Circuit Switched (CS) fallback solutions. One Circuit Switched Fallback (CSFB) solution supports not only the SMS service (Short Message Service) but also its main target, voice service. The mechanism for voice support is to fall back to a GERAN/UTRAN cell (second/third Generation network cell) that supports a circuit switched voice communication (CS voice) when the User Equipment (UE) receives or wishes to place a voice call. For Circuit Switched Fallback (CSFB) to work, a close alignment between the circuit switched (CS) Location Areas (LA) and the Evolved Packet System (EPS) Tracking Areas (TA) needs to be managed and configured in the network, based on the actual coverage areas of the cells that belong to the Location Areas (LAs) or Tracking Areas (TAs), respectively. Circuit Switched Fallback (CSFB) is considered an interim solution during the migration towards IMS (IP Multimedia Subsystem) voice.
There are reasons why Circuit Switched Fallback (CSFB), in its original form (i.e. also involving Circuit Switched (CS) voice service), is not well-suited for SMS. For example, falling back to a circuit switched (CS) capable access every time a short message is to be sent or received, seems like overkill and could break the User Equipment's (UE) Evolved Packet System (EPS) data connection every time when the User Equipment (UE) needs to move to GERAN (second generation mobile radio access network). For this reason, it has been suggested to extend Circuit Switched Fallback (CSFB) to allow sending and receiving the short messages via the Mobility Management Entity (MME) of the EPS rather than falling back onto GERAN (2G) or UTRAN (3G) functionality (i.e. performing a complete fallback to Circuit Switched (CS) functionality). To this end, the SGs interface between the Mobility Management Entity (MME) and the Mobile Switching Center (MSC), that is needed in Circuit Switched Fallback (CSFB) in order to both create, and maintain, both the attachment of the User Equipment to the Mobile Switching Center (MSC) for a User Equipment (UE) connected to or requesting a connection to the Evolved Packet System (EPS) and to perform paging for mobile terminating call, was extended to allow also the transfer of short messages to and from the UE. According to ETSI TR 21.905 V8.8.0, the term MME stands for Mobile Management Entity; according to TS23.401, V8.6.0 this term stands for Mobility Management Entity; hereinafter, the term Mobility Management Entity is used.
With such an approach, full Circuit Switched Fallback (CSFB)—i.e. including voice support—must be implemented in an operator's network even when only SMS support is desired by a User Equipment (UE). So, even an SMS-only target for Circuit Switched Fallback (CSFB) comes with the inherent disadvantage of having to upgrade all the MSCs, and to closely align the location areas (LAs) of the circuit switched (CS) network components with the Evolved Packet System (EPS) tracking areas (TAs), as this is necessary for proper association of the Mobility Management Entity (MME) with the Mobile Switching Center (MSC) per Circuit Switched Fallback (CSFB) User Equipment (UE). With another Circuit Switched (CS) fallback solution, the so-called “SMS via SGs” solution within 3GPP (Third Generation Partnership Project) (as defined in document S2-094958 of the 3GPP technical working group SA2), an SGs interface is only used to establish and maintain the association between the Mobility Management Entity (MME) and the Mobile Switching Center (MSC), and, to transfer short messages. All functionality needed for voice is eliminated, and a fallback to a CS access does not take place (to provide only an SMS functionality) since the short messages are relayed between the MSC and the UE via the MME, using the SGs interface. To enable this solution, the UE may be attached to the CS system only for SMS support. Nevertheless, this solution still has the drawback that a modified MSC is needed.